You have something in your tank that has lost it's watertight seal and has exposed an electrically live component to the water.
Rainmaker is absolutely right. This is what's referred to as a ground-fault, and it's incredibly dangerous. What I recommend in addition to his recommendations is to add a grounding probe to your system.
GFIs work by sensing leakage to ground, so you need a grounding probe to have a ground path for the current to follow. If you install a ground probe according to the manufacturers instructions, along with a GFI, using his method you should find the problem very quickly. Right now you may feel the tingle because you are grounded somewhere else, and the current may be traveling through you to get to ground. The other thing to consider is that this is not healthy for your fish, and that it could crash the tank. There was a thread not more than a couple days ago here called "Unhappy birthday to me" (or something like that) from someone whose heater failed, exposing the water to electricity, killing all the fish in his tank.
GFIs are available in several different types (I attached a picture of the two most popular styles) for 10-30 dollars.